Pair Correlations and the Electronic Structure of Neon

Using a variationally determined set of atomic orbitals, the validity of pair-correlation approximations is examined for the Ne atom. Summing the usual pair-correlation energies yields 100.5% of the correlation energy. By summing symmetry-adapted pair-correlation energies, 90.2% of the correlation energy is obtained. The complete second-order wave function which includes the Hartree-Fock configurations plus all singly and doubly excited configurations produces 88.6% of the correlation energy.